negative, you do not need a wifi extender. The Vision and non-Vision models operate on completely separate frequencies... here's a quick explanation:
P2Vision models run their RC's on 5.8Ghz, leaving the 2.4Ghz band (wifi) available for FPV and control. this is why the Vision models use actual wifi / app to do control (taking pictures, start/stop video) and fpv back to a tablet (or whatever), and you may want a wifi extender because wifi range isn't that great
P2 NON-Vision models (for gimbal+gopro) run their RC on 2.4Ghz therefore you cannot use the gopro's wifi. Furthermore, your FPV system cannot interfere so you need 5.8Ghz fpv devices... a transmitter and a receiver. Now instead of a "wifi repeater" you just need a good powerful transmitter and nice antennas and you can get vastly farther range with your FPV than wifi... like multiple kilometers basically farther than your Phantom battery will take you.
in summary, Vision models are easier but not as powerful. non-Vision is more effort and expensive to really spec out properly, but in the end you can wind up with a much more powerful and long range system.
to answer your other question about stills: you're correct since we don't have wifi control of the gopro, you have to simply hit record on the gopro before you take off... "film" the entire time, then press the button to stop after you return. There are several ways to get stills:
1) many people are happy to simply freeze frame a video and capture a still from that
2) some gopro models have a mode that captures video AND stills simultaneously (stills at certain interval)
3) put the gopro in time lapse mode and have it snapping a picture every X number of seconds... 5, 10, 30 60 whatever. so while you're flying the gopro is going snap snap snap and yes you'll have a bunch of pictures to sift through later
of note: say you're running in video + still mode on the gopro... your FPV transmission will "blink" as in the screen goes blank briefly when the camera takes a still.. this is normal. When the gopro is actually snapping a still picture specifically, it doesn't output video through the port. your video on the gopro will be normal, but the FPV feed/what you see on the screen will blank out briefly (about half a second) every time a still picture snaps... for this reason most people use a higher interval like every 10 or 30 seconds... it's kind of annoying to be honest but it's manageable. This is why most people just cap stills from the video itself so they don't have to deal with that intermittent blinking of the FPV feed.